1. Field of the Invention
The present invention relates to an f-.theta. lens system, and more particularly, it relates to a telecentric f-.theta. lens system which is applied to an optical beam scanner such as a laser printer.
2. Description of the Prior Art
An f-.theta. lens system satisfies: EQU y=f.multidot..theta.
Where y represents the distance from an optical axis to a beam spot on an image formation surface to be scanned, f represents the focal length of the f-.theta. lens system and .theta. represents the angle of incidence of the beam upon the lens system. Therefore, when a laser beam is applied to a polygon mirror, which mirror is rotated at a constant angular velocity while imaging a beam reflected by the polygon mirror on an image formation surface through the f-.theta. lens system, the beam spot is moved on the image formation surface at a constant speed. In a conventional optical beam scanner, therefore, the polygon mirror is generally combined with the f-.theta. lens system in order to move the beam spot on the image formation surface at a constant speed.
Further, the conventional optical beam scanner employs a telecentric optical system in order to prevent misregistration. This is because all principal rays enter substantially vertically an image formation surface in the telecentric optical system, to cause extremely small misregistration even if the image formation surface is displaced in the direction of an optical axis, for example, as is well known in the art.
Therefore, a telecentric f-.theta. lens system is employed in a laser printer etc. particularly requiring that a laser beam is incident upon the image formation surface with high accuracy. For example, Japanese Patent Laying-Open Gazettes Nos. 195211/1984 and 299927/1987 disclose telecentric f-.theta. lens systems of this type.
FIG. 1 illustrates the structure of a conventional telecentric f-.theta. lens system, which is disclosed in the first of the aforementioned gazettes. (Japanese Patent Laying-Open Gazette No. 195211/1984). As shown in FIG. 1, the conventional telecentric f-.theta. lens system is formed by a negative lens L.sub.12 whose concave surface S.sub.12 is directed toward an entrance pupil EP, positive meniscus lenses L.sub.13 and L.sub.14 and a positive plano-convex lens L.sub.15, and these lenses L.sub.12 to L.sub.15 are arranged in order from the entrance pupil EP side toward an image formation surface IS side. FIGS. 2A and 2B show spherical aberration and astigmatism of the telecentric f-.theta. lens system shown in FIG. 1, respectively.
A telecentric f-.theta. lens system disclosed in the latter gazette (Japanese Patent Laying-Open Gazette No. 299927/1987) is similar in structure to that shown in FIG. 1, except that the f-.theta. lens system has been corrected for chromatic aberration.
In a conventional telecentric f-.theta. lens system having the aforementioned structure, the curve of the astigmatism is generally inclined toward the negative side as shown in FIG. 2B. In order to obtain a flat image surface, therefore, it is necessary to also incline the curve of the spherical aberration toward the negative side.
However, the curve of the spherical aberration is inclined toward the positive side in the conventional telecentric f-.theta. lens system, as shown in FIG. 2A. Thus, a flat image surface cannot be obtained in the conventional telecentric f-.theta. lens system.
Further, higher resolution is generally expected from an optical beam scanner. To this end, it is necessary to reduce the diameter of a beam spot. Thus, the F-number must be minimized.